In hard disk drive systems, performance during null burst demodulation is sensitive to phase and gain error. As drive density increases, preamble lengths need to get shorter. Preamble length is a critical factor in such performance. Lower burst frequencies, such as half or ⅔ of the preamble frequency, are used to achieve higher local signal-to-noise ratios and more stable burst demodulation performance. Such low rate bursts are less sensitivity to phase error; however, this property cannot be used to reduce the total length of the preamble because a long preamble is still needed for gain estimation. Furthermore, a gain loop runs at the existing preamble field frequency, which is different from the bursts frequency. The amplitude of the burst cannot be controlled directly, which creates problems like analog-to-digital converter saturation and a need for additional normalization at the drive level.
Consequently, it would be advantageous if an apparatus existed that is suitable for using preamble fields with lower frequency for gain error calculations and repeatable runout error correction.